The present invention relates to combined cycle electric power plants, and more particularly to improved steam turbine throttle pressure limiting controls, particularly useful in the operation of a steam turbine powered by steam from one or both of two gas turbine heat recovery steam generators connected to a common supply header in such plant.
In the generation of electric power by a combined cycle plant of the type described, both gas turbines and the steam turbine may be in service, one gas turbine and the steam turbine may be in service, or only one or both gas turbines may be in service.
In such a plant, the turbine operates normally with its steam inlet control valves wide open without throttling, and with the load being governed by the rate of steam generation. The steam pressure is permitted to slide within certain limits depending on the loading of the turbine, and accepts whatever steam is generated.
The operation of a plant of this type is limited to a minimum steam pressure and flow because of the requirements of the heat recovery steam generators, and is further limited to a maximum velocity of steam to minimize erosion of the steam generator tubes and reduce the probability of water carryover into the turbine which could damage the turbine blades. At the same time, it is desirable to minimize throttling of the steam turbine control valves to maintain optimum plant efficiency and stability. This presents certain problems in that the maximum steam velocity which can be permitted depends on the steam pressure and the rate of steam generation. For example, in one embodiment, with both exhaust heat recovery steam generators in service, the rate of steam generation required no throttling of the steam turbine control valves except in maintaining the minimum required pressure for a load range of up to approximately 25%. However, with only one exhaust heat recovery steam generator in service, the rate of steam generation required throttling in the load range of up to approximately the 48% load range to maintain the minimum required pressure, and from above 70 % to 100% of the load range to limit the maximum steam velocity. The amount of throttling, of course, is also variable. For example, with one generator in service, the system can operate as low as in the neighborhood of 500 pounds pressure for a steam flow or load of approximately 70% maximum, but must operate at 600 pounds pressure for a steam flow or load of approximately 80% maximum in order to maintain the steam velocity below a predetermined maximum.
Further, in a plant of the type described the rate of steam generation can change rapidly and substantially in the event one of the steam generators is taken out of service either by operator action, or by a gas turbine or steam generator trip; and in this event the rate of steam generation decreases rapidly, which could cause excessive velocity, even though the pressure flow relationship is above that for one generator operation. Such rapid decay of pressure also would affect the shrink-swell characteristics of the water in the generator and increase the probability of water carryover into the steam turbine.
In a system for controlling the operation of a plant that includes such characteristics, it is desirable that in the automatic mode, the steam control valves are throttled to the minimum extent to maintain optimum plant efficiency, and to maintain the velocity of the steam below the required maximum for the different rates of steam generation and pressures utilized in the plant operation.
It is further desirable that such automatic control system include an analog backup system to which control of the plant is transferred in the event of a failure of the automatic control system, or in the event of certain other operating contingencies indicating a plant malfunction. One such operating contingency that is particularly desirable for overriding the automatic control is a predetermined change in the pressure flow relationship of the steam below the limits set by the automatic control system for either minimum pressure or maximum permitted velocity. In the event of this contingency, human intervention may not be able to keep the pressure flow relationship within the required limits particularly during the transfer from a two generator to single generator operation.
Therefore, in providing an analog backup control system for the steam turbine in a combined cycle plant, it is further desirable that the steam throttle pressure be controlled to decay gradually, particularly when reducing the rate of steam generation by taking one steam generator out of service, so that the pressure flow relationship of the steam flowing to the turbine is maintained within predetermined safe limits without operator intervention.
Also, in stand alone steam turbine electric power plants, which are often unattended, a similar kind of steam throttle pressure limit control can be desirable.